The objective of our work is to isolate and characterize non-structural SV40 proteins (early proteins) which may be involved in establishing and maintaining the transformed state of SV40-transformed cells. The nondefective adenovirus 2-SV40 hybrid viruses isolated by A. Lewis and coworkers are a useful tool for the study of early SV40 proteins, since late during infection host protein synthesis is shut off by the adenovirus genome. During the past year several SV40-specific proteins have been identified in cells infected by Ad2 ion ND1, Ad2 ion ND4 and Ad2 ion ND5. Cells infected by Ad2 ion ND3 do not contain detectable amounts of proteins not seen during Ad2-infection. In the coming year we intend (1) to determine the intracellular distribution, (2) to study the antigenic properties, and (3) to make peptide maps of these polypeptides. BIBLIOGRAPHIC REFERENCE: Walter, G., and H. Martin. Simian virus 40-specific proteins in HeLa cells infected with nondefective adenovirus 2-Simian virus 40 hybrid viruses. J. Virol., 16, 1236 (1975).